PURPOSE: To make a diaphragm lightweight through keeping rigidity and suitable loss by sticking collected mica foils on both sides at the same time, in foaming the mixture of flake mica, pulp and polyvinyl alcohol as a flat plate diaphragm of a speaker.

CONSTITUTION: A diaphragm 5 of a speaker is formed with a foamed body 5A obtained by the foaming of the mixture of organic or inorganic fiber such as flake mica, pulp and carbon fiber plus organic high polymer binder such as polyvinyl alcohol and epoxy, and with thin foil 5B sticked on both sides of the body 5A. The foil 5B is made of material such as mica foil and aluminum with high specific elastic rate and sticked through molten binder at foaming. At the center of the plate 5, a coil bobbin 7 wound with a voice coil 8 is fitted to constitute the speaker. With this constitution, a diaphragm having rigidity and suitable loss can be achieved, and the bending rigidity of the diaphragm is increased for light weight.